Charge dynamics in the Weyl semimetals NbIrTe$_4$ and TaIrTe$_4$ under pressure: Signatures of an electronic phase transition

This paper presents a high-pressure infrared spectroscopy and density functional theory study of Weyl semimetals NbIrTe$_4$ and TaIrTe$_4$, revealing a pressure-induced electronic phase transition at 7–8 GPa characterized by a sharp reduction in free carrier concentration and spectral weight redistribution without a significant structural change.

The Gist

The Big Picture: Squeezing Magic Rocks

Imagine you have two special rocks, NbIrTe4 and TaIrTe4. Scientists call them "Weyl semimetals." Think of these rocks not as solid, boring stones, but as electronic highways where tiny particles (electrons) zoom around without any friction or traffic jams. These highways have a special "topological" design, meaning the electrons can't get lost or crash easily.

The researchers wanted to know: What happens if we squeeze these rocks really hard?

To do this, they put tiny crystals of these materials inside a Diamond Anvil Cell. Imagine a tiny, high-tech vice made of diamonds that can squeeze a speck of dust with the pressure of a mountain range. They squeezed these rocks while shining infrared light (like a super-powered flashlight) through them to see how the electrons reacted.

The Discovery: The "Tipping Point"

The scientists found that as they increased the pressure, nothing much happened at first. But then, they hit a specific "tipping point" at about 7 to 8 GigaPascals (GPa) of pressure. (For context, this is about 70,000 to 80,000 times the pressure of the atmosphere at sea level).

At this exact moment, the rocks underwent a phase transition. It's like water suddenly turning into ice, but instead of freezing, the electronic behavior of the rock changed completely.

What Changed? (The "Traffic Jam" Analogy)

Before the pressure reached that tipping point, the electrons were flowing freely, like cars on an open highway. The rock acted like a very good conductor of electricity.

After the tipping point, two major things happened:

1. The Traffic Slowed Down: The number of free-moving electrons dropped sharply. It's as if the highway suddenly developed a massive construction zone, and the "free flow" of traffic was blocked. The rock became less "metallic" and more resistant to the flow of electricity.
2. A Hidden Sound Emerged: Before the pressure, the free-flowing electrons were so loud (so dominant) that they drowned out a quiet "hum" or vibration inside the rock (a phonon). It's like trying to hear a whisper in a stadium full of screaming fans. Once the pressure squeezed the electrons into a slower, less dominant state, the "screaming fans" quieted down, and the researchers could finally hear the "whisper" (the phonon vibration) that was there all along but hidden.

Was it a Structural Break or an Electronic Shift?

When you squeeze something hard, you might expect it to physically break or change shape (like crushing a soda can). The researchers checked for this using a technique called Raman scattering (which is like listening to the rock "sing" when hit with light).

• The Result: The rock didn't crack or change its basic shape. The "song" it sang changed slightly in pitch, but the structure remained the same.
• The Conclusion: This wasn't a physical breakage; it was an electronic makeover. The arrangement of the electrons inside the rock rearranged itself, even though the rock's skeleton stayed the same.

The Computer Simulation (The "Digital Twin")

To understand why this happened, the scientists used supercomputers to build a "digital twin" of the rocks. They simulated squeezing the digital rocks and watched what happened to the electron highways.

• The Simulation Confirmed: The computer showed that the "electron pockets" (the areas where electrons live) started to shrink and break apart.
• The Cause: The pressure squeezed the layers of the rock closer together. Think of the rock as a stack of sticky notes. At normal pressure, the notes are slightly apart. When you squeeze them, the "sticky" forces between the layers get stronger. This change in how the layers interact forced the electrons to rearrange their paths, causing the "traffic jam" and the sudden change in behavior.

The Takeaway

This paper tells us that by simply squeezing these special rocks, we can tune their electronic personality. We can switch them from a state where electrons zoom freely to a state where they are more restricted.

The researchers found that this change happens at the same pressure for both types of rocks (NbIrTe4 and TaIrTe4), suggesting a universal rule for how these materials behave under pressure. It proves that pressure is a powerful tool to reshape the invisible electronic world inside these materials without breaking them.

Technical Summary

Technical Summary: Charge Dynamics in Weyl Semimetals NbIrTe4 and TaIrTe4 Under Pressure

Problem Statement
Weyl and Dirac semimetals host topologically protected, gapless electronic excitations. The ternary transition-metal iridium tellurides, NbIrTe4 and TaIrTe4, are proposed type-II Weyl semimetals characterized by tilted Weyl cones and, in the case of NbIrTe4, potentially complex configurations of Weyl points and nodal lines. While high-pressure studies have previously identified superconductivity in these materials at high pressures and hinted at electronic phase transitions at lower pressures, the precise nature of these low-pressure transitions remains unresolved. Specifically, previous studies on NbIrTe4 suggested a transition between 4 and 12 GPa involving Fermi surface reconstruction, yet lacked consistent results regarding whether this transition is structural or electronic. Furthermore, the behavior of TaIrTe4 under similar low-pressure conditions was largely unexplored, with existing research focusing primarily on high-pressure superconductivity. A comprehensive understanding of how hydrostatic pressure tunes the interlayer van der Waals coupling to drive electronic changes, and direct spectroscopic evidence of topological feature evolution, was lacking.

Methodology
The authors investigated the charge dynamics of single-crystal NbIrTe4 and TaIrTe4 under hydrostatic pressure using a multi-faceted approach:

• Experimental: Infrared reflectivity measurements were performed over a broad frequency range (170–18,000 cm⁻¹) using a diamond anvil cell (DAC) with CsI as a pressure-transmitting medium. Data were collected up to ~13.5 GPa. The reflectivity data were analyzed via Kramers-Kronig relations to extract the complex optical conductivity. The spectra were modeled using a Drude-Lorentz fit to separate free-carrier (Drude) contributions from interband transitions and phonon modes.
• Complementary Measurements: High-pressure Raman scattering measurements were conducted under identical experimental conditions to detect potential structural phase transitions.
• Theoretical: Density Functional Theory (DFT) calculations were performed using the VASP code with the PBEsol functional. The study included the calculation of pressure-dependent electronic band structures, Fermi surfaces, density of states (DOS), and interband optical conductivity using the Kubo-Greenwood formula and maximally localized Wannier functions.

Key Results

1. Critical Pressure and Spectral Weight Redistribution: Both materials exhibit a distinct anomaly at a critical pressure $P_c \approx 7–8$ GPa. Above this pressure, a significant redistribution of spectral weight occurs in the optical conductivity. Specifically, there is a sharp reduction in the low-frequency Drude response (free carrier conductivity) accompanied by an enhancement of interband optical conductivity in the 250–2000 cm⁻¹ range.
2. Emergence of a Phonon Mode: A low-energy phonon mode near ~220 cm⁻¹, which is screened by free carriers at lower pressures, emerges as a distinct feature in the reflectivity spectrum above $P_c$. This indicates a reduction in the free carrier concentration.
3. Nature of the Transition: Raman scattering measurements show changes in mode positions and splittings near $P_c$ but lack the pronounced discontinuities or mode splitting patterns characteristic of a first-order structural phase transition (such as the $T_d \to 1T'$ transition seen in WTe2). This suggests the transition is predominantly electronic in nature, though subtle second-order structural distortions cannot be entirely ruled out.
4. Drude Plasma Frequency: The Drude plasma frequency ($\omega_p$) exhibits a sharp decrease above $P_c$ for both compounds. Since the total density of states at the Fermi level ($E_F$) remains relatively constant in DFT calculations, this reduction is attributed to topological changes in the Fermi surface (fragmentation and shrinking of electron/hole pockets) rather than a loss of electronic states.
5. Interband Conductivity and Nodal Lines: The interband optical conductivity ($\sigma_{1,inter}$) shows a plateau-like behavior at higher energies, consistent with the presence of energy-dispersive nodal lines. Under pressure, the level of this plateau increases, suggesting an elongation or expansion of the nodal lines, consistent with theoretical predictions for TaIrTe4.
6. Theoretical Corroboration: DFT calculations confirm that the pressure-induced changes in optical conductivity arise from topological modifications of the Fermi surface (Lifshitz-type transitions) and an accumulation of Nb 4d and Ir 5d states in the conduction band, which increases the joint density of states for low-energy excitations.

Significance and Claims
The paper provides collective evidence for a reversible, pressure-induced phase transition in both NbIrTe4 and TaIrTe4 at $P_c \approx 7–8$ GPa. The authors claim that this transition is most likely electronic, driven by a reconstruction of the Fermi surface (a Lifshitz transition) rather than a major structural change. The study highlights that hydrostatic pressure effectively tunes the van der Waals interlayer coupling, driving these systems through a topological electronic transition. This work establishes pressure as a key parameter for manipulating the electronic band structure and topological features of these Weyl semimetals, offering new insights into the interplay between interlayer interactions, Fermi surface topology, and charge dynamics in topological materials. The findings are consistent with previous transport data indicating a shift from multiband to hole-dominated conduction in NbIrTe4 and provide a unified spectroscopic picture for both compounds under similar experimental conditions.